Energy Resolved Phonon Scattering in Glasses

[摘要] Amorphous materials posses low temperature thermal properties which are different from those of crystalline materials but are common to a wide range of disordered materials. The low temperature heat capacity is larger than that of the corresponding crystalline material and below 1K has a linear term in temperature as well as the T3 term. In this temperature range the thermal conductivity is lower tha that of the crystalline material and proportional to T2. At temperatures between 1 and 10K it is approximately constant. Below 1 K the behaviour of these materials can be understood in terms of the two level system model. However at higher temperatures none of the various theories proposed to account for the observed behaviour have been confirmed experimentally. This is due to the insensitivity of measurements like those of thermal conductivity to the exact form of the energy dependence of the phonon mean free path and to whether the scattering is elastic or inelastic. Thus, to clarify the theoretical picture, a direct determination of the energy dependence of the phonon mean free path as a function of energy is required. Energy resolved phonon scattering measurements were performed using a heat pulse technique. Two generators were deposited on one face of a sapphire crystal substrate and two phonon detectors were deposited on the opposite face. A thin film of glass was deposited under either one of the generators or one of the detectors. The phonon generators were thin film constantan heaters and the detectors were superconducting tunnelling heterojunctions. The ratio of the currents generated in the detectors by the phonon fluxes with and without the glassy film were calculated at different biases. By comparing these to current ratios calculated from theoretical models the phonon mean free path as a function of energy could be deduced. The results of these measurements led to the following conclusions. Thin film constantan heaters produce a phonon spectrum which can be described by a simple acoustic mismatch theory. The phonon scattering in sputtered silicon dioxide is best explained by elastic scattering with a mean free path of 2.2mum at 1meV and varying as E-6. This form of scattering fitted the data measured from three samples. Two of these were under a detector and the third had been deposited under a phonon generator. However inelastic scattering cannot be ruled out. In evaporated silicon monoxide evidence of two scattering mechanisms was observed. A strong, probably elastic, process which saturated as the phonon input power was increased. At higher powers this gives way to a weaker process. A scattering mechanism with a significant dwell time was observed in sputtered arsenic trisulphide.

[发布机构] University:University of Glasgow

[关键词] Condensed matter physics, Low temperature physics